However, what Democrats and political forecasters are really looking at when saying that the Republican Party is “shrinking” are the demographics of the parties’ composition.

The Republican Party has a lock on rural, white-majority working class areas and older voters generally, but the long-term prospects of those electorates aren’t great. America is diversifying, becoming more educated, ever-moving into cities and suburbs, and the older, conservative generations will be replaced by liberal ones.

So even if you can kinda-sorta point to long-term measures of voter identification, it’s not really any particular poll that Democrats and others cite to say that the Republican Party is shrinking, but long-term demographic trends on top of their voting behavior.